1. Field of the Invention
The present invention relates to a tympanostomy tube and more particularly to an antimicrobial tympanostomy tube designed to reduce the occurrence of post-operative otorrhea following myringotomy frequently encountered with the insertion of such tubes in the ear.
2. Description of the Prior Art
The use of tympanostomy tubes for the treatment of otitis media with effusion is the most commonly performed surgical procedure in the United States. Children with persistent middle ear effusions who do not respond to antibiotics undergo a procedure in which a myringotomy is performed in the tympanic membrane under general anesthesia.
In this procedure an incision is made in the tympanic membrane, fluid from within the middle ear is aspirated and a tympanostomy tube is inserted. The tubes can have various configurations and materials, and are effective in correcting the hearing loss due to the effusion as long as the tubes are in place in the ear. The materials which can be used to make tympanostomy tubes include thermoplastics such as modified elastomers and olefins, thermosets such as silicone and polytetrafluoroethylene; and metals such as stainless steel and titanium.
Purulent otorrhea frequently develops after tube insertion. In one study by H. G. Birck and J. J. Mravek xe2x80x9cMyringotomy for Middle Ear Effusions,xe2x80x9d Ann. of Otol. Rhino. Laryngo., volume 85, pages 263-267 (1979), the investigators observed that 15% of children having tympanostomy tubes inserted in their ears following myringotomy developed postoperative otorrhea. In a more recent study by George A. Gates et al, xe2x80x9cPost Tympanostomy Otorrhea,xe2x80x9d Laryngoscope, volume 96, pages 630-634, (June 1986), the investigators observed that the incidence of tympanostomy tube induced otorrhea following myringotomy was 18%. In a clinical study performed by Balkany et al, xe2x80x9cA Prospective Study of Infection Following Tympanostomy and Tube Insertion,xe2x80x9d American Journal of Otology, volume 4, pages 288-291 (1983), the investigators observed an incidence of postoperative otorrhea of 19% in children receiving tympanostomy tubes with no antibiotic drops postoperatively applied. In the Balkany et al study, the investigators found that the incidence of postoperative otorrhea was reduced to 6% when antibiotic drops were put into the patient""s ear after myringotomy. In another study on the use of antibiotics after myringotomy, R. S. Baker and R. A. Chole, xe2x80x9cA Randomized Clinical Trial of Topical Gentamicia After Tympanostomy Tube Placement,xe2x80x9d Arch. Otolaryngology Head and Neck Surgery, volume 114, pages, 755-757 (July 1988), the investigators found that the incidence of infections in the experimental group using Gentamicin, an ophthalmic solution used as otic drops, had an incidence of infection significantly reduced by antibiotic drops.
In both the Balkany et al and Baker et al studies using antibiotic drops after tympanostomy, the investigators used potentially ototoxic antibiotics, namely Cortisporin and Gentamicin. Based on their frequency of use, and the lack of adverse effects noted in these studies, antibiotic drops are now used routinely to prevent postoperative otorrhea. However, thorough studies demonstrating the absence of adverse toxicological reaction in the use of antibiotic drugs for the treatment of postoperative otorrhea have not been published.
In addition to the relatively high incidence of otorrhea after myringotomy, investigators have observed children with implanted tympanostomy tubes sometimes experience bouts of otorrhea. Occasionally, the otorrhea became persistent causing some investigators to believe that the tympanostomy tubes become colonized with pathogenic bacteria.
The relatively high incidence of otorrhea after myringotomy and tympanostomy tube insertion exposes patients with persistent middle ear effusions to significant morbidity and additional treatment time and cost.
It would be desirable to utilize tympanostomy tubes whereby the incidence of otorrhea and other microbial induced infection after myringotomy and tympanostomy tube insertion could be substantially reduced without the use of antibiotics and the potential ototoxic reaction associated with the use of such drugs.
One approach for reducing bacterial infection encountered with the use of medical devices inserted into body cavities has been to apply an antimicrobial coating to the surface of the medical device. For example, U.S. Pat. No. 4,592,920 to Murtfeldt; U.S. Pat. No. 4,603,152 to Laurin et al and U.S. Pat. No. 4,677,143 to Laurin et al each teach applying a coating containing an antimicrobial agent such as silver oxide to the outer surfaces of medical devices such as catheters, enteral feeding tubes, endotracheal tubes and other hollow tubular devices.
U.S. Pat. No. 4,592,920 to Murtfeldt is primarily concerned with providing a surface coating of an antimicrobial metal compound on a medical device such as a catheter, but also discloses that the metal compound can be xe2x80x9cimbeddedxe2x80x9d within the entire catheter. However, the Murtfeldt patent finds the imbedded construction to be less desirable since the antimicrobial metal compound imbedded within the side wall of the catheter has less likelihood of encountering migrating microbes and by inference is less effective than a surface coating.
In accordance with the present invention, it has been found that tympanostomy tubes formed from a microporous, highly flexible thermoplastic or thermoset resin having a high gas permeability rate and having about 0.5 to 15% by weight of a silver compound incorporated throughout the sidewall of the tympanostomy tube exhibits sufficient antimicrobial activity to alleviate or prevent postoperative bacterial infections normally associated with the use of these tubes and without adverse toxicological reaction with otologic tissue such as irritation or inflammation.
The tympanostomy tube of the present invention is characterized by its biological compatibility with otologic tissue associated with the middle ear and contains silver additives, such as silver oxide incorporated therein which are capable of migrating ions or other formed compounds to the surface of the tube sidewall to impart a therapeutically effective amount of antimicrobial activity to the sidewall surface. Moreover, the release of antimicrobial agents from the tube sidewalls proceeds at a rate adapted to provide long term prophylaxis for the prevention of post-operative otorrhea following myringotomy with the insertion of tympanostomy tubes, thereby reducing the need for the prophylactic administration of antibiotics.
Dispersing the silver oxide throughout the tube instead of using a surface coating is preferred for a number of reasons, including:
1. There is significant difficulty associated with coating a product the size of an ear ventilation tube. Tube geometries typically are cylindrical in shape with abrupt diameter changes and odd shaped flanges. The tube geometry and small tube size make it difficult to control coating thickness. Dispersing the metal oxide homogeneously throughout the tube assures an even distribution of the metal oxide throughout the tube including the inner lumen area.
2. It is also believed that the metal oxide will be functional over a longer period of time when dispersed throughout the tube, rather than as a coated product since a tube material such as silicone with its high permeability rates will provide pathways for the oxide to migrate to the surface and provide antimicrobial activity for a period of time longer than a surface coating.
3. It is easier to control the amount of the silver oxide present in the tube when it exists as a homogeneous dispersion throughout the tube, compared with a silver oxide coating on the surface of the tube. This is because a change in tube thickness varies the oxide contact and the thickness of the coating is difficult to control.
The antimicrobial tympanostomy tube of the present invention can be prepared by mixing a suitable microporous elastomeric resin having a relatively high gas permeability rate, for example about 26xc3x9710xe2x88x929 cubic centimeters (cm3) of air through one square centimeter (cm2), surface area membrane 1 centimeter thick in 1 second at 1 centimeter of Hg pressure, with a concentration of silver additives or compounds ranging from about 0.5 to about 15% by weight of the resin.
To insure thorough and uniform dispersion of the silver containing additive or compound in the resin, the resin is preferably in the form of a paste and the silver compound in the form of particles having an average particle size of about 5 to about 100 microns, which are milled thoroughly to facilitate uniform dispersion of the silver compound in the resin paste.
The milled product is then formed into a hollow tube by any conventional tube forming process such as transfer molding, extrusion or casting. Once formed, the tube is cured to its final form by baking the tube in the mold or in an oven at a temperature sufficient for curing the resin paste. The product is packaged and sterilized before shipment, as for example with ethylene oxide in a manner which is well known to those skilled in the art.
Resins which are microporous and have the requisite high gas permeability properties can be used to prepare the antimicrobial tympanostomy tubes of the present invention. These resins are characterized by an ability to transmit or leach antimicrobial silver ions. The resins include curable silicones, PVA""S, thermoplastic elastomers, acrylonitrile-butadiene-styrene copolymer rubber, and polyurethanes. Curable silicone resins are preferred for the manufacture of the antimicrobial tympostomy tubes of the present invention due to their molecular structure which provides good flexibility both microscopically and macroscopically, and high gas permeability rates.
Silver compounds which provide the antimicrobial silver ions and which can be incorporated in the sidewall of the antimicrobial tympanostomy tube of the present invention include silver oxide, silver chloride, silver iodide, silver acetate, silver citrate, silver nitrate, silver sulfadiazine, and silver sulfate, with silver oxide being preferred.
In the examples which follow, all parts and percentages are by weight unless otherwise indicated.